1. Field of the Invention
The invention relates to a pressure-reducing valve and a controller for a blow-molding machine and method thereof. Specifically, the invention relates to a pressure reducing valve for reducing the pressure supplied to a head tooling hydraulic positioning valve and a controller for bypassing the pressure reducing valve in a blow-molding machine and method for regulating the pressure of a cylinder that controls the position of a head tooling pin and die assembly in a blow-molding machine.
2. Description of Related Art
A mandrel pin and a die arrangement, commonly referred to as a head tooling, are used in an extrusion blow-molding process to form a molten tube of plastic called a “parison.” The parison is extruded from an extrusion head into a hollow mold that is in the shape of the desired product. Once the parison is captured by the mold, the mandrel pin and the die are closed to prevent excess plastic from flowing from the extrusion head. Air is then injected into the captured parison causing the parison to conform to the shape of the hollow mold. After sufficient cooling time, the mold is opened and a formed piece of plastic is removed.
An inner diameter of the die and an outer diameter of the mandrel pin determine the desired size and shape of the parison that is to be produced by the blow-molding machine. Adjusting a die gap opening or clearance between the mandrel pin and the die varies a wall thickness of the finished part. A hydraulic cylinder is coupled to the mandrel pin or the die tooling and is used to adjust the die gap opening between the mandrel pin and the die. The cylinder that controls this movement is generally called a programming cylinder.
The programming cylinder must have sufficient force to precisely adjust the die gap opening of the mandrel pin and the die during the extrusion of the parison. Depending on the chosen machine controller and parts to be produced, the die gap opening can be adjusted in excess of 100 times during the formation of the parison. To provide a rapid and a precise movement of the mandrel pin and the die against the flow of parison, a hydraulic pressure of at least 2,000 psi is often required by the programming cylinder.
Several products of differing sizes and shapes are produced by blow-molding machines and often require the use of several varieties of the pin and die toolings. A machine “setup technician” must manually calibrate the die gap opening of the mandrel pin and the die for each new pin and die tooling. The calibration can be performed electrically by adjusting a position of the programming cylinder. If this adjustment is not carefully preformed, the mandrel pin and the die can come into contact with each other. If contact occurs between the mandrel pin and the die, the high hydraulic pressure at the programming cylinder can damage the mandrel pin, the die, and/or other extrusion head components.
The industry lacks an accurate and safe apparatus and method for preventing accidental contact between and damage of the mandrel pin and the die tooling that is caused by an inaccurate calibration of the position of the programming cylinder in a blow-molding machine.